They dont leech off the drop rates of event girls they just have different rates. If you do 20 battles the system will randomly pick each game for normal prizes or affection number. The math is pretty basic to see if you have more girls you can collect then they will basically get in the way of progress of a limited time girl. Yes it is still possible to collect any girl of rarity with the right among of time without spending but the need to spend will be higher if you are low on time and with normals included in the rng will probably drive up the cost of getting them with coins. For example, I aquired the legendary Easter Bunny in a round 4 day about the time of legendary days without spending but it would probably took me more time or some coins if I had normals included in the rng to get her.